1. Field of the Invention
The present invention relates to an optical recording and/or reproducing apparatus, and more particularly, to a liquid crystal device for compensating for aberration, an optical pickup including the liquid crystal device, and an optical recording and/or reproducing apparatus employing the optical pickup.
2. Related Art
Generally, in an optical recording and/or reproducing apparatus for recording and/or reproducing information to and/or from an optical disc, that is, an optical information storage medium using a beam spot to which an objective lens focuses a laser beam, the recording capacity of an optical disc is determined by the size of the focused beam spot, which is proportional to the wavelength λ of a laser beam and inversely proportional to the numerical aperture (NA) of objective lens. Equation (1) defines a beam spot size S that is determined by a wavelength λ of a laser beam used and a numerical aperture (NA) of the objective lens:S∝λ/NA   (1)
Therefore, to reduce the beam spot size S for increasing the optical disc's recording density, it is necessary to use a shorter wavelength light source such as a blue laser and an objective lens with NA higher than 0.6.
Since the emergence of a CD technology designed to record and/or reproduce information using light of 780 nm wavelength and objective lens with NA of 0.45 or 0.5, much research has been conducted to increase data storage capacity by increasing the area recording density, and has resulted in a DVD technology designed to record and/or reproduce information using light of 650 nm wavelength and objective lens with NA of 0.6 or 0.65.
Currently, studies are now underway to develop a next-generation high density optical disc system that can offer over 20 GB recording capacity using blue light of a predetermined wavelength, e.g., 405 nm. Standards on high density optical discs are still actively under development, some of which are almost finalized.
The standards specify use of blue light of 405 nm wavelength and an objective lens with NA of 0.65 or 0.85 that will be described below. The standards also set a thickness of a DVD disc to 0.6 mm that is 50% less than 1.2 mm of a CD disc. This is intended at providing a tolerance due to a tilt of the optical disc since NA of objective lens in DVD standards increases to 0.6 that is higher than 0.45 of objective lens in CD standards.
Furthermore, if the NA of the objective lens is increased to 0.85, for example, for a high density optical disc with storage capacity higher than that of a DVD, the thickness of the high density optical disc must be reduced to about 0.1 mm. A blu-ray disc (BD) standard is a recording technology that increases NA of an objective lens while reducing the thickness of an optical disc in this way. A BD standard specifies a 405 nm wavelength light source, 0.85 NA objective lens, and about 0.1 mm thickness of an optical disc.
The thickness of an optical disc refers to a distance between an incident surface and a recording surface. For CD or DVD, the thickness of an optical disc is approximately equal to a substrate thickness. For BD, the thickness of an optical disc is approximately equal to the thickness of a cover layer.
It is known that a margin for an optical disc thickness error allowable in a BD system currently available is small value of 3 μm. Spherical aberration produced when a deviation of the thickness of the optical disc exceeds the margin may significantly degrade the quality of recorded and/or reproduced signal.
Therefore, an optical element for compensating for spherical aberration contained in an optical signal being recorded and reproduced is needed for an optical disc such as a BD with a small allowable thickness difference. In particular, a high density optical disc system using a cover layer with a thickness of approximately 0.1 mm and an objective lens with a NA of 0.85 highly requires an optical element for compensating for spherical aberration induced when the thickness of the optical disc deviates from the standard.
An optical pickup compatibly applying a CD, a DVD, and a high density optical disc also requires an optical element for compensating for chromatic aberration induced by a difference between wavelengths emitted by a light source as well as spherical aberration caused by a difference between thicknesses of the optical discs. Here, the high density optical disc is a blu-ray disc (BD) or a high definition digital versatile disc (HD DVD). The HD DVD uses a light source with 405 nm wavelength and an objective lens with 0.65 NA. The thickness of a substrate is 0.6 mm like in the DVD.
Conventionally, a liquid crystal panel is used to reduce and compensate for spherical aberration by creating a phase difference distribution of an opposite sign to spherical aberration introduced by an optical pickup by applying an electric field. The liquid crystal panel is connected to a driving circuit by lead wires.
Such an aberration-compensating liquid crystal panel is formed by homogeneous rubbing, that is, a process to align liquid crystal molecules parallel to a substrate before applying an electric field. An indium tin oxide (ITO) electrode is patterned to correspond to the distribution of aberrations. Spherical aberration is corrected by applying an electric field to the conventional liquid crystal panel to generate a phase difference of an opposite sign to spherical aberration.
One conventional approach to ITO electrode patterning involves patterning an ITO film into electrodes and depositing thin lines of higher conductivity electrodes on edges of the patterned ITO electrodes. An example of a liquid crystal panel provided with patterned ITO electrodes for compensating for spherical aberration is presented in the Japanese Laid-open Patent Application 2004-110959.
However, a method of manufacturing the conventional liquid crystal panel is complicated. This is because a process of patterning metal line electrodes is additionally required, after patterning the ITO electrodes to obtain electric field distribution that is coincident with the distribution of spherical aberrations. In addition, an individual lead wire and a driving circuit that separately operates are needed for each patterned electrode.
The liquid crystal panel with a patterned electrode structure for compensating for aberration further requires individual interconnect wire and driving circuit for driving each patterned electrode, thus leading to even more complicated interconnection and driving structure.
Furthermore, the structure in which metal line electrodes are added after formation of patterned ITO electrodes cannot achieve compensation of spherical aberration at a small interval between each patterned electrode. This also results in reduction in transmittance of light through the metal line electrodes.